The initial focus of this project is to examine the expression of cyclin mRNA's during cell proliferation in hepatocytes. As a first step, a reverse transcriptase/polymerase chain reaction (RT/PCR) assay was developed to detect cyclins A and B1 mRNA's. The expression of these two cyclins was determined in vivo in rat livers 24 hr after a two-thirds partial hepatectomy and compared to primary hepatocyte cultures stimulated with insulin and epidermal growth factor (EGF). High expression of both cyclin A and B1 mRNA's were detected in regenerating liver using the RT/PCR assay. Primary hepatocyte cultures undergo DNA synthesis between 24-48 hr after addition of EGF. Between 36-48 hr, the labelling index was >50% for insulin and EGF treated cultures and <5% with insulin alone. The mitotic index in both groups was <5%. The expression of cyclin A and B1 mRNA's, in both groups, was virtually undetectable at the time of plating and by 36 to 48 hr increased to comparable levels found in regenerating liver. Present results indicate that cyclins A and B1 are important during the G2/M phase of the cell cycle but are not sufficient to induce mitosis.